Generally, a motor vehicle powershift transmission includes a number of gear elements coupling the input and output shafts, and a related number of clutches which are selectively engageable to activate gear elements for establishing a desired speed ratio between the input and output shafts. The clutch may be of the band or disk type.
For example, the input shaft may be connected to the engine through a fluid coupling, such as a torque converter, and the output shaft is connected directly to the vehicle drive. Shifting from one gear ratio to another involves releasing or disengaging the off-going clutches associated with the current gear ratio and applying or engaging the on-coming clutches associated with the desired gear ratio.
Transmissions of this type are often electronically controlled and include diagnostic functions for determining when a fault condition exists. For example, diagnostic systems can sense electrical problems with the control solenoids. A system of this type is illustrated in U.S. Pat. No. 4,414,863 issued Nov. 15, 1983 to Heino. Diagnostic systems have also been developed which place a transmission control into one of two states depending on the type of fault condition detected. Such a system is described in an SAE Technical Paper entitled "Adaptive Electronic Tractor Shift Control System" dated Sep. 10-13, 1990 by Larry F. Ross and Dale W. Panoushek. In one state, a speed shift change from the current state is not allowed. In this case, either a shifter signal or clutch pedal switch signal was determined to be invalid, and the controller keeps the transmission in the last speed prior to the invalid condition. In the second state, the transmission control disengages all clutches. In this case, the controller detected an invalid condition in either a solenoid circuit or a clutch pressure signal. The control module deenergizes all speed solenoids in order to avoid simultaneous clutch engagement due to ambiguous input states.
Such a system does not fully address the need to maintain transmission operation when possible, but if necessary, prevent a shift into a gear not requested by the operator and prevent clutch tie-up. Further, the prior art system does not fully address the need to associate groupings of electrical and mechanical failure types with the appropriate set of available gears to maintain transmission operation when possible.
The present invention is directed to overcoming one or more of the problems as set forth above.